I think your right there are a lot of simplification in play in photography that break under some circumstances.

If you check that MIT slide deck I linked there is some interesting material on simplifications and covers off both distance shooting and indeed macro shooting where its also busted

I know that there is a slight error in the formula because the circle size is always based on the focus distance error as a fraction of the correct focus distance which is always assumed to be the focal length. That's close enough most of the time but for 1:1 (i.e. true macro) the lens to subject and lens to image distances are both 2xf so the formulae for near and far point [ HD/(H-D) and HD/(H+D) ] don't work properly. I've got a vague memory that you use D+F instead of D and when F is 50mm and D is 5M the error really doesn't matter. When D is 100mm ....